1. Field of the Invention
The present invention relates to an air-fuel ratio detecting device and a method and, particularly, to an air-fuel ratio detecting device which correctly and very precisely detects the air-fuel ratio of an internal combustion engine based upon the characteristics of each air-fuel ratio sensor and each air-fuel ratio sensor circuit.
2. Description of the Related Art
There has been known a linear air-fuel ratio sensor which is disposed in the exhaust system of an internal combustion engine (hereinafter referred to as an engine), detects the air-fuel ratio of the engine from the exhaust gas of the engine and generates an output which varies in proportion to the air-fuel ratio that is detected. In a device for controlling the air-fuel ratio by feedback by using the air-fuel ratio sensor, a map for calculating the air-fuel ratio of the engine in response to the output of the air-fuel ratio sensor is formed in advance through a bench test, the formed map is stored in a storage circuit, the air-fuel ratio of the engine is calculated from the map and from the output of the air-fuel ratio sensor mounted on the real engine, and the air-fuel ratio of the engine is so controlled by feedback as to approach the stoichiometric air-fuel ratio which most purifies the exhaust gas.
However, the processing circuit (hereinafter simply referred to as the air-fuel ratio sensor circuit) for processing the output of the air-fuel ratio sensor for forming the map used for the bench test, is different from the air-fuel ratio sensor circuit that is really used for the engine. Therefore, the air-fuel ratio that is really detected from the engine does not serve as a correct value. According to a method of correcting the output of an oxygen concentration (air-fuel ratio) sensor of an internal combustion engine disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-214249, the data of a map for calculating the air-fuel ratio of the engine in response to the output of the air-fuel ratio sensor, are corrected based on the output of the air-fuel ratio sensor. Though an error in the characteristics of the air-fuel ratio sensor has been taken into consideration, however, an error in the characteristics of the air-fuel ratio sensor circuit has not been taken into consideration. Therefore, though the output characteristics of each air-fuel ratio sensor are corrected, the idea is based on a prerequisite that no error is contained in the output at the stoichiometric air-fuel ratio; i.e., the output corresponding to the stoichiometric air-fuel ratio is not corrected.
According to the method of correcting the output of the air-fuel ratio sensor of an internal combustion engine disclosed in the above Japanese Unexamined Patent Publication (Kokai) No. 62-214249, the output of the air-fuel ratio sensor corresponding to the stoichiometric air-fuel ratio is set to be the same for all air-fuel ratio sensors without taking into consideration an error that exists in the air-fuel ratio sensor circuit. Therefore, the air-fuel ratio of the engine is not correctly and precisely detected, lacks reliability in controlling the air-fuel ratio by feedback and makes it difficult to purify the exhaust gas of the engine to a high degree.